This invention relates to a television signal selection device in a television receiver for automatically searching for a proper video signal or for promptly leading a received signal into a proper state.
FIG. 1 is a block arrangement diagram of a television signal receiving set. This receiving set comprises an antenna 1, an RF amplifier 2, a local oscillator and mixer 3, a PLL circuit and prescaler 4, a microcomputer for channel selection 5, an intermediate circuit (a SAW filter 6, an adjacent voice trap 7, and a video signal demodulator 8), a video signal processor 9, a CRT 10, and a video signal detector 13. Numeral 11 designates an AFT voltage output which is applied from the video signal demodulator 8 to the channel selecting microcomputer 5, and numeral 12 a video signal detection output which is provided from the video signal detector 13. The SAW filter 6 and the adjacent voice trap 7 constitute a SAW filter with an adjacent voice trap characteristic.
FIG. 2 shows the characteristics of an AFT voltage (a voltage for automatic reception frequency tuning) and a television signal detection output versus a transmission carrier frequency deviation in a television receiver which has a system (hereinbelow, termed "pseudo-synchronous demodulation system") hitherto often used wherein a video carrier is sampled by a resonance coil to perform the quadrature demodulation of a video intermediate-frequency signal. In the above system, the resonance characteristic of the video carrier is comparatively gentle. Therefore, even for transmission frequency deviations in, for example, the HRC or ICC format of CATV broadcast in U. S. A., the optimum reception state could be established in such a way that the PLL circuit and prescaler 4 shown in FIG. 1 was controlled using a decision at the reception of a regular reception frequency as stated below. Assuming in FIG. 2 that a frequency range for search is a regular transmission frequency .+-.2 MHz, a received television signal is presumed to lie in a range from a deviation -f.sub.1 to +2 MHz if the television signal detection output at the reception of the point of a transmission frequency deviation of zero is "high," and it is presumed to lie in a range from the deviation -f.sub.1 to -2 MHz or to be quite nonexistent if the detection output is " low." Once the television signal has been detected, it can be thereafter led to the optimum reception frequency according to the characteristic curve (or S-shaped characteristic curve) of the AFT voltage versus the transmission carrier frequency deviation. This operation is illustrated in a flow chart of FIG. 4. If the television signal is detected upon the selection of the regular reception frequency f.sub.o by the PLL circuit, the received frequency is raised from the previous frequency every .DELTA.f along the path of a loop l.sub.1. In contrast, if the television signal is not detected, the received frequency is lowered every .DELTA.f along the path of a loop l.sub.2. Thus, it is possible to arrive at the optimum reception state in which the television signal exists and in which the AFT is in an M mode.
FIG. 3 shows the frequency correlation of video, chroma and sound carriers in a transmitted television signal, and the overall response characteristic of the video intermediate-frequency circuit of a television receiver receiving the transmitted signal, including a lower-side adjacent voice signal trap.
The television receiving set in the prior art is constructed as described above, and can search for the television signal comparatively easily. However, it has had the problem that a long time is required to determine an absence of a proper video signal in a frequency band for search or to determine when a received signal is extremely spaced from the regular reception frequency. Meanwhile, requests for a higher picture quality and a higher tonal quality have recently been made suddenly, and a system (hereinbelow, termed "complete synchronous demodulation system") is adopted wherein the video carrier for the quadrature demodulation is generated using a PLL in the video signal demodulator 8 shown in FIG. 1. Since, however, the lock-in range of the PLL exists in this case, the prior-art system illustrated in FIGS. 2-4 has been incapable of appropriately receiving the television signal in a case where the television signal lies in a frequency region departing from the lock-in range and where the frequency region is a frequency region to be searched.